The field of this invention relates to temperature compensated magnetic bearing systems and more particularly to such bearing systems for supporting vertical rotors of induction-type watthour meters.
Magnetic bearing systems are well-known for vertically supporting the rotor of an induction meter of the type including watthour meters. The rotor is rotatably supported along a vertical axis by the bearing system so as to maintain a predetermined gap spacing between two magnet assemblies so that a clearance is maintained between a disc carried by the rotor and an electromagnetic portion of the meter stator. These bearing systems include stationary and suspended permanent magnets oriented to produce interacting repulsion or attraction magnetic forces to maintain the meter rotor at the desired vertical position. Consistent with the rugged and compact design of watthour meters and their extended continuous use in varying temperature and ambient conditions, the magnetic bearing systems must be compact, easily manufactured and assembled in mass production, and further must be highly accurate, reliable and maintenance free. One substantially universal consideration of the magnetic bearings is temperature compensation. Most permanent magnet bearing materials are substantially temperature sensitive and the magnetic strengths change inversely with changes in temperature. A reference intermediate bearing gap spacing is typically established at a room temperature of approximately twenty-four degrees C. The size of the permanent magnets, the associated pole face areas and the strength of the permanent magnets' fluxes are critically determined to support the rotor through the reference gap. A portion of the total permanent magnet fluxes is diverted from the gap at the room temperatures and at higher temperatures less flux is diverted so that the magnet sizes, pole face areas and magnet strengths must be proportionally greater than that which be required for rotor support at a constant ambient room temperature.
It has also been observed that increasing the diameter of the magnet pole faces to increase the gap spacing or to provide for further temperature compensation also increases the susceptibility of the bearing system to unbalanced magnetic flux distributions tending to cause greater radial magnetic forces and an undesired side thrust on the rotor shaft. The bearing systems normally require that the bearing support fluxes be symmetrical about the axis of rotation of the rotor being suspended. It has also been observed that in bearing systems having a critical minimum gap spacing that not only are the permanent magnet dimensions and temperature compensating designs critical but also the characteristics of the permanent magnet material must be constant and uniform so that each bearing system is substantially identical for use in supporting rotors of identical meters. It has been found that sometimes the characteristics of the permanent magnet materials can vary due to variations in the constituent materials and the manufacturing processes used to make the permanent magnet materials. Thus, the materials do not always have strictly constant magnetic characteristics to provide bearing magnets which have the maximum flux producing properties desired.
The present invention is most closely related to the repulsion type of permanent magnet bearing systems disclosed in U.S. Pat. Nos. 3,143,704 issued Aug. 4, 1964 to D. F. Wright; 3,309,152 issued Mar. 14, 1967 to Ramsey et al; and 3,810,683 issued May 14, 1947 to J. M. Keever et al, all assigned to the assignee of this invention and incorporated herein by reference. The bearing systems described and claimed in the aforementioned patents include upper and lower permanent magnet assemblies having hollow or ring-shaped permanent magnets carried within soft magnetic cups in facing relationship so as to form the repulsion or floating type of system. The magnetic cups form series flux return paths for the pole face fluxes and are circularly disposed outward of the permanent magnets. The lips of the cups define magnetic poles which are opposite from that of the adjacent magnet pole faces and materially increase the magnetic flux available to lift the meter rotor. The magnetic cups are also effective to substantially decrease leakage fluxes and to produce a more uniform flux pattern at the pole faces. The Keever et al patent discloses a modified form of the bearing system including a temperature compensating sleeve disposed in the center of the lower permanent magnet assembly. The last-named patent extends the temperature range for operation of the bearing system which has a minimum critical gap spacing in the order of 0.018 inch (0.046 cm.). The bearing systems described in the aforementioned patents are highly successful, however, if substantial deviations occur in the characteristics of the permanent magnet materials the permanent magnets do not have a sufficiently strong support producing interaction for extended time periods. In extreme temperature conditions changes in gap spacing may become too great for the minimum critical gap spacing required in the aforementioned highly efficient and compact design.
Other prior art patent disclosing face-to-face or repulsion-type permanent magnet bearing systems are U.S. Pat. Nos. 2,254,698; 2,315,408; 3,107,948; 3,326,610; 3,370,896; and 3,657,676. The aforementioned patents do not include a magnetic bearing system having facing permanent magnets carried in soft magnetic cup members and further including temperature compensating elements in both the upper and lower permanent magnet systems as included in the present invention. The U.S. Pat. Nos. 3,325,757 and 3,325,758 both disclose temperature compensated magnetic structures having upper and lower facing permanent magnets each including a temperature compensating sleeve around the outer diameters rather than the inner diameters. The last two named patents are intended for producing a constant magnetic flux in the space between the magnets and are both stationary so as to not provide a magnetic bearing system.